“Although mass violence committed by people with mental illness is horrifying and galvanizes public attention, most instances of multiple murder are not perpetrated by people with mental illness, and only around 4 percent of the violence in this country is attributable to mental illness,” says Paul Appelbaum, director of the division of law, ethics, and psychiatry at Columbia University in New York, in an e-mail response to questions.
The vast majority of even those individuals with a serious psychiatric disorder never commit violence of any sort, he adds, and “in the absence of a previous history of violence, which has been true for most of the recent mass shooters, it is almost impossible to determine who will commit violence and who will not.”
Under federal law, individuals who have been involuntarily committed to a mental institution or “adjudicated as a mental defective” are prohibited from possessing firearms, but relatively few people – including Rodger and the perpetrators of the Sandy Hook and Aurora movie theater shootings – fall into that category.
California law goes further: Individuals are temporarily banned from gun possession if they’re placed on a psychiatric hold or if they’ve made a specific threat to an identifiable person to a therapist, who is then required to report the threat to law enforcement.
But none of those instances applied in the case of the Isla Vista shooting.